a) Field of the Invention
This invention relates to a process for preparing acrylamide by subjecting acrylonitrile to catalytic hydration with water in the presence of a copper-base catalyst. More specifically, this invention is concerned with a purification process for acrylonitrile useful as a raw material for the preparation of high-quality acrylamide which permits production of a polymer having sufficiently high molecular weight and also good water-solubility.
b) Description of the Related Art
Acrylamide has long been used in the form of acrylamide polymers as papermaking chemicals, flocculants, oil recovery agents and the like and has also found wide-spread utility as a raw material comonomer for various polymers. As a preparation process of acrylamide for such applications, the so-called sulfuric acid process was used in the past but in recent years, catalytic processes featuring a reaction in the presence of a copper-base catalyst have been developed and are now industrially practiced in place of the sulfuric process.
Among the above-described applications of acrylamide, flocculants in particular have found utility expanded to the treatment of effluent and the like in recent years. Keeping step with this, a great deal of efforts are now under way for the improvement of their quality and performance. Of these, there is a marked tendency toward high-molecular acrylamide polymers for use as flocculants because they are said to exhibit better performance as their molecular weights become higher. Recently, those having a high molecular weight of 10,000,000 or higher, especially of about 15,000,000 are demanded. Compared with the fact that the molecular weight required for an acrylamide polymer or other polymers employed for other applications is generally 1,000,000 or lower, this molecular weight is far higher. In addition, the acrylamide polymer so obtained is required to promptly dissolve in water without leaving any insoluble matter behind because it is usually employed as a flocculant in a form dissolved in water. In view of the toxicity of acrylamide monomer, the unreacted monomer contained in each polymer is required to be in a trace amount, for example, not greater than 0.2 wt. %.
These requirements are contradictory to the demand for higher molecular weight so that tremendous efforts have been made to achieve them. Although such high-molecular acrylamide polymers constitute only one example of applications of acrylamide, they cannot be furnished for a wide variety of general applications unless they are suited for application as flocculants. A process according to the present invention relates to the preparation of acrylamide which can be furnished for such an application.
The term "molecular weight" as used herein means a molecular weight determined by the testing method shown in Example 1 which will be described subsequently herein. Water solubility takes significance usually when a polymer obtained in an aqueous medium is dried into dry powder having a water content of 20 wt. % or lower, especially 10 wt. % or so. The term "water solubility" as used herein is primarily employed in this sense.
To produce an acrylamide polymer having such high molecular weight and sufficient water solubility as described above, it is considered important to pay attention not only to a process for the production of the polymer but also to the quality of acrylamide. Further, the quality of acrylonitrile as the raw material is also considered to significantly affect the production of such an acrylamide polymer.
Acrylonitrile is usually synthesized by ammoxydation of propylene. Acrylic fibers and ABS resin account for a majority of applications of acrylonitrile. Acrylonitrile, which is employed as a raw material for the production of acrylamide by the catalytic hydration process, is required to have higher quality so that it contains less impurities than that employed for such major applications. To meet this requirement, it is the common practice to take a measure such as making operation conditions for a distilling purification step severer.
To make acrylonitrile suitable for the catalytic hydration process, several methods have been proposed.
According to Japanese Patent Laid-Open No. 118305/1988 (corresponding to U.S. Pat. No. 4,177,210), for example, acrylonitrile as a raw material is brought into contact with an H-type cation exchange resin to lower the content of oxazole to 200 ppm or less, more preferably to 25 ppm or less in the acrylonitrile. This publication also discloses that acrylamide, which has been synthesized by subjecting the acrylonitrile to hydration in the presence of a copper-base catalyst, has higher stability and when polymerized, provides an aqueous solution of higher viscosity compared with acrylamide synthesized likewise from oxazole-containing acrylonitrile. It is also disclosed that as a method for the regeneration of the cation exchange resin, the cation exchange resin is brought into contact with hot water, water vapor, methanol, a slightly-acidic aqueous solution or a mixture thereof.
Japanese Patent Publication No. 26264/1982 discloses that when acrylonitrile is purified by an inorganic acid or an acidic cation exchange resin, the acrylonitrile can avoid deterioration of the catalytic activity compared with unpurified acrylonitrile in hydration reaction in the presence of a copper-base catalyst.
Japanese Patent Publication No. 26586/1982 discloses that when the concentration of acrolein in acrylonitrile is reduced to 1.5 ppm or less, more preferably 0.8 ppm or less by reacting the acrolein with acetylacetone or the like and then by isolating the reaction product and the acrylonitrile from each other by distillation or the like, acrylamide obtained by subjecting the acrylonitrile to hydration in the presence of a copper-base catalyst can provide a polymer having good water solubility.
Japanese Patent Publication No. 1108/1983 discloses that acrylamide--which has been obtained by lowering the content of acrolein to 0.8 ppm or less in acrylonitrile by bringing the acrylonitrile into contact with a porous ion exchange resin having primary and/or secondary amino groups as exchanging groups and then subjecting the resultant acrylonitrile to hydration in the presence of a copper-base catalyst--can provide a polymer having good water solubility and sufficiently high molecular weight.
Similarly, Japanese Patent Laid-Open No. 134063/1983 discloses that when the content of aldehydes, practically acrolein in acrylonitrile is reduced by bringing the acrylonitrile into contact with a gel-type, weakly-basic ion exchange resin containing primary and/or secondary amino groups, the acrylonitrile can form acrylamide capable of providing a polymer having improved water-solubility and also that when acrylamide itself is treated in a similar manner, the acrylamide so treated can provide a polymer having improved water-solubility and sufficiently high molecular weight.
According to Japanese Patent Publication No. 9303/1986, acrylonitrile is subjected to water extraction and/or to water extraction and distillation to lower the content of acetonitrile to 20 ppm or less, more preferably 10 ppm or less and is then subjected to hydration at a temperature of 100.degree.-140.degree. C. in the presence of a copper-base catalyst. It is disclosed that this acrylamide can provide a polymer having improved water solubility.
Further, U.S. Pat. No. 2,444,589 issued in 1948 points out that acrylonitrile synthesized from an inorganic cyanide and an organic substance contains trace amounts of ionic impurities and trace amounts of neutral impurities and these impurities hamper isolation of a synthetic reaction product from acrylonitrile as a raw material and lower its yield. It is also disclosed that when this acrylonitrile is treated with a cation exchange material (for example, a phenylformaldehyde condensation product, sulfonated coal or the like) and an anion exchange resin (for example, a condensation product of guanidine, urea or formaldehyde) to remove ionic substances and is then treated with a decoloring agent (for example, activated carbon), decoloration not feasible by single use of the ion exchange materials or the decoloring agent is feasible. Further, it is also disclosed in one of its examples that acrylonitrile--which has been treated successively with a cation exchange resin, an anion exchange resin and activated carbon in the order that they are presented--provides a high polymerization velocity.
According to a finding of the present inventors, the quality of acrylamide obtained by subjecting acrylonitrile to catalytic hydration with water in the presence of a copper-base catalyst cannot provide an acrylamide polymer sufficient in water solubility and molecular weight when polymerized singly or with another comonomer, even if oxazole, acrolein, acetonitrile and the like in the acrylonitrile are eliminated by using the above-described art.
The process commonly practiced these days, in which operation conditions for the distillation-dependent purification step are made severer to prepare low-impurity acrylonitrile for use as a raw material for the preparation of acrylamide, involves considerable sacrifices of a recovery loss in the distillation-dependent purification step and increased consumption of energy such as steam. Although use of so-obtained low-impurity acrylonitrile in catalytic hydration generally leads to acrylamide of good quality, the product may be obtained with insufficient quality in some instances. Accordingly the process is insufficient in the stability of quality and is insufficient as a commercial preparation process for high-quality acrylamide.
At the time of the issuance of U.S. Pat. No. 2,444,589 in 1948, said U.S. patent containing the above-mentioned example directed to the use of the cation exchange resin, the anion exchange resin and activated carbon, neither (a) a preparation process of acrylonitrile by ammoxydation of propylene nor (b) a preparation process of acrylamide by hydration of acrylonitrile in the presence of a copper-base catalyst had been practiced industrially. This U.S. patent does not contain any disclosure about (c) purification of acrylonitrile prepared by ammoxydation of propylene by the process of the patent or (d) effects of acrylamide, which was obtained by catalytic hydration of acrylonitrile (c) with water in the presence of the copperbase catalyst, on an acrylamide polymer obtained by polymerizing the acrylamide singly or with another comonomer. Further, neither the purification (c) nor the effects (d) have been known to date.
Although in the above-mentioned example, acrylonitrile was treated with the cation exchange resin, the anion exchange resin and activated carbon in the order that they are presented, what is disclosed in this patent is to easily eliminate neutral impurities as a cause for coloration or the like by removing ionic substances. In other words, it is only necessary to perform the cation exchange treatment and the anion exchange treatment prior to the treatment with the decoloring agent (for example, activated carbon). The above U.S. patent does not specify the order of the cation exchange treatment and the anion exchange treatment in the ion exchange treatment.